Display devices, such as liquid crystal displays (LCD's), twisted nematic (TN) have recently come into extensive use in display systems used on various types of products ranging from computers to military equipment including displays in airplanes, tanks and vehicles and military hardware. These displays have come to wide spread usage due to their low maintenance requirements, and generally high reliability. More specifically, their wide spread use in military applications has come about due to the standardization and reliability of manufacturing techniques used to manufacture such displays.
Although liquid crystal and other types of displays have been in use for a number of years now, problems are now beginning to show up due to various usage conditions, and the length of time that the displays have been in use.
For example, it has recently been discovered that some liquid crystal displays can exhibit voids or "white spots" in the liquid crystal material after being subject to certain combinations of environmental extremes. In addition, some displays which are formed by two plates of glass or other similar material bonded together and sealed to enclose the liquid crystal material have exhibited failures in the seal itself. Careful investigation by the present inventors has lead to the discovery that such failures are due to ultraviolet light energy affecting the liquid crystal material, the seal, and thin films in the display. In addition, further investigation has disclosed that shock, either an immediate impact on the display, thermal, or a more remote vibrational type shock in the vehicle or hardware to which the display is mounted can also cause such results.
It was well known in the prior art to attempt to protect the displays from ultraviolet light sources. It was believed that ultraviolet light will damage the displays, over time, however no concrete data on the extent and manifestation of the problem was available. It was believed, from various test results, that prior art display systems would exhibit some degradation after 20 to 25 hours of exposure to ultraviolet light energy.
Accordingly, some prior art display systems attempted to solve the anticipated problems of exposure to ultraviolet radiation by providing ultraviolet coatings or filters in conjunction with the display system. These solutions were not adequate, however, for several reasons. First, although some of the prior art coatings and filters promised a high absorption of ultraviolet energy under 400 nanometers and a high transmission of visible energy over 400 nanometers, most coatings and filters actually exhibited a cut off in absorption at approximately 375 nanometers. Accordingly, ultraviolet energy in the range of 375 to 400 nanometers was not filtered or absorbed and thus, caused degradation, over time, with the display system.
Secondly, other coatings or filters absorbed and filtered the visible spectrum of light up to approximately 410 nanometers or higher. This caused a sharp loss of color in the display system, along with the problem of light reflecting off the display system.
Thirdly, thin film ultra-violet (UV) coatings affect other depositions and processes such as High Efficiency Anti-Reflective coatings that are also very angular dependent, and pass increased UV doses depending on the incident angle
An additional method which was utilized in the prior art was to provide plastic or acrylic display covers or display elements. The problem with plastic or acrylic displays is that they cannot be used with the present manufacturing procedures which require high temperatures to bond various elements together. In addition, plastics and acrylics are highly susceptible to scratching even with hardcoatings.
A further problem which is present in both filter or coating prior art methodologies and the acrylic or plastic display methodologies are that some elements used to provide ultraviolet filtering flouresce or shine at night when stimulated with various shorter wavelengths. According to military specification MIL 85762A, also commonly referred to as NVIS (night vision imaging system) specifications, the chemical composition of any coatings, filters or other elements destined for military applications cannot shine or "fluoresce" at night. This makes the military NVIS hardware useless, potentially rendering a pilot or driver "blind" while using NVIS equipment.
An additional problem with the plastic and acrylic materials are that these materials tend to change color or yellow over time.
Many of the problems with the prior art and present day display systems were recently manifested in the extreme environmental conditions which were present during recent Military operations commonly referred to as Operation Desert Storm in the Middle East. The cockpits of airplanes and other military hardware were exposed to extremes of environmental conditions including light, temperature, and thermal and mechanical shock.
Aircraft and tanks, for example, have a high vibration profile. In addition, usage of these displays in military applications during operational readiness, often makes the display susceptible to impact and shocks from other military hardware, including seat belts, etc.
In yet another attempt at providing ultraviolet light protection from display systems, some prior art methods could include providing an ultraviolet coating on the cockpit windows of airplanes or other large glass areas on military hardware. Again, the same problems expressed earlier apply to such large areas of glass or plastic and are aircraft type dependent.
Although there have been some discussions in the past with utilizing ultraviolet light absorbing glass and the manufacturing of display systems, those glasses have generally not been usable for a number of reasons. For example, glass that has a high alkaline content such as standard soda lime glass must generally be passivated before it can be used in the manufacture of liquid Crystal displays in order to prevent some of the ions from leaching out and having an affect and reacting with some of the elements and coating used to manufacture displays. Additionally, some glass or glass type materials are not compatible with present day manufacturing and display processing techniques due to various reasons such as lack of high temperature capabilities.
Accordingly, what is needed is a glass type material which can be used in conjunction with present day manufacturing and display system processing techniques which utilize high temperatures and require a glass of relatively low alkalinity. Most importantly, what is needed is a glass with a high ultraviolet light blocking ability up to and including approximately 400 +/-10 nanometers from light incident at an angle without affecting the amount of blocked UV light, but with a very sharp cut off of absorption at and above 400 nanometers, to allow essentially all of the visible light to be transmitted above 400 nanometers. Additionally, such glass or glass like element must have an ion and elemental content which does not fluoresce under NVIS conditions, and which has a thermal co-efficient of expansion (TCE) which closely matches that of other elements used in the display manufacturing process.